1. Field of the Invention
The present invention relates to drilling and milling bits and more particularly discloses a drill bit and method of making the same by converting the point of conventional drill bits to a point configuration better suited to drilling KEVLAR.RTM. fiber laminates and similar composite materials incorporating high strength fibers. (KEVLAR is a registered trademark of the DuPont Chemical Co.).
2. State of the Prior Art
The point of presently used drill bits is generally cone shaped and normally includes two cutting edges which slope rearwardly from a leading center point in a V shaped configuration. As the drill bit rotates and advances against the workpiece surface, initial contact is made at the center of the drill by the leading point. Material is, therefore, first removed from the center of the hole area such that the cutting action proceeds from the center outwardly towards the edge of the hole. There is, to some extent a wedging action as the drill point advances and a resultant tendency for material to be pushed radially outwardly by the drill point.
In a solid workpiece such as a block of metal, such cutting action is effective because the rigidity or resistance offered by the workpiece material to the advancing drill bit is generally the same at all points on the hole area; that is, the material last removed by the drill near the edge of the hole is still solidly affixed to the remainder of the workpiece body and yields to the cutting edge by separating from the workpiece.
The situation is different where laminates made of resin impregnated textile material, such as woven Kevlar fibers, are drilled. As the V shaped point of the drill advances into the fiber matrix, an ever increasing number of individual fibers are severed at the center of the hole, leaving free ends which are supported only by other adjacent fibers with which the severed fibers are interwoven. As the area of the hole increases as the drill advances into the textile, the number of such free ends increases rapidly. These free ends are relatively loosely supported within the fiber matrix and do not offer sufficient resistance to be severed by the cutting edges of the drill. Instead, the fiber ends at the edge of the hole yield to the V shaped cutting edge and are pushed outwardly, without being cut, to allow passage of the drill bit body. When the drill bit is withdrawn, the fiber ends return to their natural positions, extending into the freshly cut hole along its entire circumference. The result is that the effective diameter of the hole is somewhat smaller than that of the drill bit used and has a ragged edge with strangling filaments. Both the entry hole and exit hole of bores cut through Kevlar by standard drill bits are characterized by uncut lose filaments with rough bore hole surfaces. This phenomenon is known in the industry as "swelling" of the material. The bore hole is not cut to the dimension of the drill bit, but will always appear to be undersized or slightly smaller than the outer diameter of the drill bit. Such dimensional inaccuracies are undesirable, particularly in critical aerospace structures where Kevalar laminates and related composite materials find ever increasing application.
The known prior art includes so called spur bits which have a pair of radially outer points or spurs and edges connecting the points to a center spur of the drill bit. In particular, applicant is aware of drill bits sold under the mark Woodbits by the Leichtung Inc. Co., located at 4944 Commerce Parkway, 1079 PS Cleveland, Ohio 44128. These Woodbits are described as having a center spur that prevents skating of the bit, two cutting spurs that start and dimension a perfectly round hole, and extremely sharp flutes. These bits, however, are claimed to be designed specifically for wood, and further make use of conventional fluting rather than the parabolic flute preferred by the applicant for use with Kevlar and similar laminates. In addition, applicant's drill point configuration improves over the prior art in the curvature of the cutting edges which are designed to pull material radially inwardly as it is severed by the outer cutting points and further improve over the prior art in that the angle of the helix in the vicinity of the cutting point is reduced so as to avoid "grabbing" of the material and encourage a shearing action instead.
While the problem has been described in connection with the drilling of Kevlar, it is characteristic of many machining operations of fiber materials and fiber composites.